Conventionally, injection molding apparatus comprises a clamping base, a pair of upstanding support panels provided at opposite ends of the clamping base, a guide member which extends horizontally between the support panels, a movable mold which is slidably movable along the extending direction of the guide member, a fixed mold which is fixedly mounted on one of the support panels as opposed to the movable mold, and a mold-driving cylinder which is fixedly mounted on the other one of the support panels to slidingly move the movable mold toward and away from the fixed mold.
Generally, in performing injection molding with the above-constructed apparatus, the movable mold is pressingly moved toward the fixed mold along the extending direction of the guide member to be clamped with the fixed mold with a sufficient clamping force, and then, melted resin is injected in the cavity defined by the movable mold and the fixed mold. Then, the injected resin is cooled and solidified to produce a molded article.
In the above conventional injection molding method, it is highly likely that a large residual stress remains in a molded article. With such a large residual stress remaining in a molded article, it is highly likely that strain or refractive index in the molded article exceeds a permissible range if such a molded article is to be used for special purposes such as parts for precision instruments and optical instruments.
In producing molded articles for special use, an injection compression-molding method has been proposed in an attempt to produce molded articles free of residual stress (see Japanese Unexamined Patent Publication No. 2000-6231, Japanese Unexamined Patent Publication No. 4-228298). According to this method, before injection molding, a pair of molds are set to such an intermediate clamped state where the molds are spaced apart from each other with a small clearance, or the molds are clamped to each other with such a moderate clamping force as to allow the molds to be spaced apart from each other under an injection pressure. Then, the molds are clamped to each other again to a final clamped state upon injection of melted resin into the cavity defined by the molds.
However, the above injection molding apparatus has been designed based on the idea of using ordinary injection molding method of injecting melted resin in a state that the molds are set to a final clamped state where the molds are clamped to each other with a sufficient clamping force. Accordingly, the technology disclosed in the above publications does not consider necessity of keeping the parallelism of the movable mold and the fixed mold in a condition that melted resin be injected in an incomplete clamped state.
Consequently, in the case where molded articles are produced by the above injection compression-molding method, there rises a problem, as shown in FIG. 9, that parallelism of parting surfaces of molds 91, 92 is not maintained due to, for example, flexure of a guide member 93, with the result that quality of molded articles may be deteriorated.
Further, if the molds 91, 92 are set to a final clamped state by re-clamping in a state that sufficient parallelism between the molds 91, 92 is not secured, it is highly likely that the molds 91, 92 may be damaged or broken due to abrasion between the molds 91, 92.
The above-mentioned drawbacks become remarkable in the case where mechanical wear of the injection molding apparatus such as abrasion of the guide member 93 and tilting of support panels 94, 95 becomes serious by repeated operations of the injection molding apparatus.